Synthesis of highly non-stoichiometric Cu2ZnSnS4 nanoparticles with tunable bandgaps

Non-stoichiometric Cu₂ZnSnS₄ nanoparticles with average diameters of 4–15 nm and quasi-polyhedral shape were successfully synthesized by a colloidal method. We found that a non-stoichiometric composition of Zn to Cu in Cu₂ZnSnS₄ nanoparticles yielded a correlation where Zn content increased with a decrease in Cu content, suggesting formation of lattice defects relating to Cu and Zn, such as a Cu vacancy (V_Cu), antisite with Zn replacing Cu (Zn_Cu), and/or defect cluster of V_Cu and Zn_Cu. The bandgap energy of Cu₂ZnSnS₄ nanoparticles systematically varies between 1.56 and 1.83 eV depending on the composition ratios of Cu and Zn, resulting in a wider bandgap for Cu-deficient Cu₂ZnSnS₄ nanoparticles. These characteristics can be ascribed to the modification in electronic band structures due to formation of V_Cu and Zn_Cu on the analogy of ternary copper chalcogenide, chalcopyrite CuInSe₂, in which the top of the valence band shifts downward with decreasing Cu contents, because much like the structure of CuInSe₂, the top of the valence band is composed of a Cu 3d orbital in Cu₂ZnSnS₄.